Oxygen or water vapor gasification of biomass, such as wood, peat, straw or logging waste, can generate gas which comprises hydrogen approximately 35-45% by volume, carbon monoxide 20-30% by volume, carbon dioxide 15-25% by volume, methane approximately 8-12% by volume, and nitrogen 3-5% by volume. It is possible to use this gas as, for example, a synthesis gas of diesel-category fuels. Steam/oxygen gasification of biomass can be an interesting alternative economically, for example, when the scale of operation is large enough.
Problems associated with gasification can include, for example, variations in gas composition and amounts of impurities. It is possible to purify gasification gas efficiently from tar impurities and ammonia which are contained in it by using catalysts at a high temperature. Examples of catalysts which are suitable for decomposing tar are nickel catalysts and dolomites, the operating temperatures of which can be at minimum 800-900° C. For example, gasification technology is disclosed by Pekka Simell, Catalytic hot gas cleaning of gasification gas, VTT Publications 330, Espoo 1997.
A zirconium catalyst (FI patent 110691), which has been developed by VTT Technical Research Centre of Finland, also works relatively efficiently in decomposing tars, for example, heavier hydrocarbons. In addition, the zirconium catalyst enables the use of a considerably wider temperature range than does a nickel catalyst, for example, a temperature range of 600-900° C.
When using nickel catalysts, the high temperature employed can present a problem. Use of such high temperature can form soot (coke) during the process of the catalytic gas conditioning. The coking problem can be made worse in applications of synthesis gas, in which light hydrocarbons (for example, methane) are intended to be reformed as efficiently as possible. In this case, the metal catalysts, for example, nickel, are used at very high temperatures (950 to 1100° C.). The generation of soot causes accumulations of carbon deposits on the catalysts and the reactor, and may eventually result in clogging the whole reactor.
At the start-up of the gasification process, the use of nickel or other metal catalysts presents problems because the temperature in the catalytic unit is relatively low, for example, below 700° C. During the start-up, the operation of the gasifier may occasionally be unstable, and the tar content of the product gas may then occasionally rise extremely high. These conditions may together cause an accumulation of carbon on the nickel catalyst and clogging of the catalyst reactor, and accelerate deactivation of the nickel catalyst.
A catalytic reformer, which is used in the purification of gasification gas, can be heated by using partial oxidation (partial combustion) of the gas in a position before the catalyst bed or in the catalyst bed, in which case the process is called an “autothermal reforming.” After the gas is oxidized, its temperature increases considerably, in which case also the number of the thermal, i.e. coking, side reactions increases.
It is possible to reduce the coking of the metal catalyst in the reformer by using phased reforming. Phased reforming means that the reforming is carried out in several stages, for example, several sequential reaction zones, in which two or more catalysts are used.
According to FI Patent Application No. 118647 (Method for reforming a gas containing tar impurities, inventors: P. Simell and E. Kurkela), in the first stage of a phased reformer (“pre-reforming stage” or “pre-reformer”), a zirconium catalyst is used. While the gas is being partly oxidized in the zirconium catalyst, the heaviest tar compounds are decomposed into gas components. Almost no carbon is generated in the zirconium catalyst and, consequently, no carbon blockage of the reactor takes place.
However, results of the trial runs which were carried out show that the use of a zirconium catalyst in the pre-reformer does not always reduce the generation of coke adequately. This applies in cases where very high temperatures (for example, over 900° C.) are employed in the secondary stage. Such occasions occur for example in applications of synthesis gasification in which a nickel catalyst is used at high temperatures for the actual reforming.
In conditions such as these, to ensure the functionality of the process, it can be desirable to prevent the generation of coke in the first catalyst layers (pre-reforming stage).